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I’ve recently been having a go at removing some of the surface rusting on the parts of my car I can’t or don’t want to completely replace and thought I would share the results.

After using a citric acid dip for de-oxidising small parts with great success I wondered if the same could be done on parts of the body shell. I have been preparing a poultice to hold a citric acid on the metal, it seems to work quite well.
My approach is loosely based on a cleaning technique used for removing sulphate soiling on historic fabric (heavy deposits of black dirt on stone buildings).

The mix is as follows

Small bucket with a half a kettle of boiled water
Between 5% to 20% citric acid poured into hot water
Bag of cellulose pulp, added to water and citric acid an mixed until it forms a (quite wet) soft workable mix
A small amount of sepiolite powder (clay powder) just added as a plasticiser for workability
The poultice can then be applied to the rusted areas, it doesn't need to be a thick coating, around 1/4"- 3/8" seems to hold the moisture well. After application cover with cling film to stop it from drying out, also I find it more workable when covered in cling film.
As this is experimental I'm not sure how long to leave it, I tend to add more citric acid (around 10%-20%) and this speeds up the process. Around 3-4 hours gives this result. There is a tiny bit of rust in some of the deeper pits as can be seen so a second application is needed here.
I don't think citric acid is harming the steel but I'm very thoroughly washing off with water and a small wire brush afterwards.
I'm guessing also its getting pulled into the overlapped panels with a capillary action so I am washing out and blowing wet/dry with compressed air. It needs drying after washing anyway as flash rusting happens pretty quickly.
I found a phosphate steel prep holds the steel rust free if you don't want to paint straight away, like rustbusters phosclean B or POR15 metal ready. I'm not so keen on these, they cost more money than I want to spend and they leave salts all over the surface that are more difficult to wash off, probably impossible to wash out of overlapped panels as well.

I've been wiping out most of the rust across the bulkhead area like this, some of the rust was quite stubborn requiring several applications and I think one or two more may be needed to completely clean.
It takes a fair while but I like to have a go myself and have more time than money so suits my needs.

Just to add I have been checking the cleaned steel with a micrometer after de rusting, if its lost more than about 0.3mm of its original thickness then I have cut out the offending area and repaired by butt welded in a localised repair

Well done


looking good.

Pioneering stuff....well done for sharing this
Thank you

Great stuff, I've wondered about doing this myself. I use citric acid in a bath too, and really like it.

Tim

Tim wrote:I use citric acid in a bath too, and really like it.

Tim

Doesn't it sting a little?

999 ORX wrote:

Tim wrote:I use citric acid in a bath too, and really like it.

Tim

Doesn't it sting a little?

its the only thing that will shift his 'naughty rash'

A little, but I'm completely free of rust!

Tim

999 ORX wrote:

Tim wrote:I use citric acid in a bath too, and really like it.

Tim

Doesn't it sting a little?

Depends where you put it... The mind boggles...

Saw something about cheap vinegar soak.... Then bicarbonate soak to neutralise...
Anyone tried that?

Citric is very cheap, you won't save much by using vinegar. I rinse in hot water, then neutralise using a detergent type of degreaser/parts washer which is mildly caustic. If you don't the surface flashes over with a fine coating of rust fairly quickly.

Tim

Yes, like Tim says citric acid is very cheap.
I had a very quick try with vinegar but I found it much slower than citric acid. With citric the strength can be easily adjusted, also if the solution becomes weak just add more. Much easier than vinegar and less smelly.
For small parts you can de rust on the cooker with a large pot, citric acid works really quickly when its hot.
I've bought a couple of 1KG bags, I think it was less than £10. I'm still on the first bag and have made several smallish dipping baths and some poultices.

Another fantastic tip Ollie! You are truly a pioneer!

M

Thanks for the compliments but really its not pioneering at all.
Lots of people are using a citric acid bath for oxide removal like Tim, I just mixed it up with some paper pulp and threw it on the car. Its an idea I had while at work as a poultice using the paper pulp, sepiolite, water and ammonium carbonate is used by building conservators to break down soiling.
Bilt Hamber sell a product called deox C, a powder/crystals to add to water for dipping rusty parts, there's a clue in the name. I've never used deox C but I have used Bilt Hambers deox gel, its very good but I suspect its just citric acid with methylcellulose (wallpaper paste). Similar idea but at around £10 a litre, probably about 10% citric acid you soon see how much can be saved making your own. I think a poultice is much better than a ready mixed gel, it holds the moisture well, is easier to seal with cling film and you can adjust the strength to suit.

Glad I could contribute something to this excellent site

Cheers
Ollie

This sounds like a great idea, I use brick acid for removing corrosion from components, but I've never tried it as a poultice, maybe citric acid is better, but where would I go to obtain it? I guess I may have to squeeze many lemons

I got mine from ebay. They sell it to ladies to make lovely smelling fizzy 'bath bombs'. Its used very widely as a food additive as well.

Once it is used up you are left with horrible black looking stuff that smells slightly sweet. Apparently it is very good for your garden. You can by chelated iron at the nursery to green up your lawn, or make it in a bucket while de-rusting your mini.

Tim

I think brick acid is hydrochloric acid, I've tried it at 10% I guess brick acid is around that strength.
We have some Hydrochloric at work and use it for de scaling the inside of a high pressure cleaning machine. It does work well but it won't stop at the rust, if left too long it will (slowly) attack the steel.
I would say citric is better fo de oxidising yes, the problem with hydrochloric is getting at it to neutralise. Like your using it, ok on small parts but not ideal on the car if it gets into the seams, it might cause problems later on? Maybe.
For a poultice I would recommend citric. Like Tim I got my bags of citric on ebay.
I have used it prior to media blasting as well. I have a pair of clubman wings I cut from a car and they had some surface rust on around the seams so I did some experiments. I put some citric on the night before and blasted them the next day, I think the rusts grip was weakened as it came away easily using a very soft media.
Also I found clubman wings just about fit in the bath for dipping but I did have a job explaining why the bath had been stained green.

Citric works by chelating the iron, in effect it converts it into an organic compound which dissolves easily in water. It can chelate metallic iron, but it works much faster on iron oxide. If you leave the part in for 24 hour or so it won't bother the base metal, but if you forget the part, it will eventually be eaten away.

These two were a pair. The piston on the left was taken out after 24 hours, but I missed the one on the right until I went to do the rebuild about a month later...I had to buy a new slave cylinder to get another piston.



Tim

[quote="Tim"]I got mine from ebay. They sell it to ladies to make lovely smelling fizzy 'bath bombs'. Its used very widely as a food additive as well.

Thanks Tim, I'll check that out, I would never have thought of looking in that section of ebay!